As you know, you need to get the contents of the launch file into a Qt for display. I'd think that the easiest way to do this would be to create your QStringList before the process.start line, then 'send' it to the MainWindow (by whatever means you'd like).
For instance -- and note that this is just a sketch, not implemented/tested -- you might have a QTextEdit widget as a member of the MainWindow. After MainWindow creation, you could simply do a MainWindow->QTextEdit->setText(QStringList.join("\n")).
@dornhege, if one wants to be able to run everything in a single Qt executable, starting a roslaunch process is a decent option -- although in this case, because the launch file selection isn't embedded in the MainWindow and thus can't be chosen via Qt widget, I'd agree with you.